Writing instrument



FIG. 1 is a front, right perspective view of an ornamental design for a writing instrument according to the present invention with the barrel illustrated in broken lines;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a back elevational view thereof;

FIG. 4 is a left side elevational view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a top end view thereof; and,

FIG. 7 is a bottom end view thereof.

All portions shown in broken lines are environmental and illustrative only and form no part of the claimed design. 

The ornamental design for a writing instrument, as shown and described. 